Pediatric heart transplant (HTx) recipients have reduced exercise capacity typically two-thirds of predicted values, the mechanisms of which are not fully understood. We sought to assess the cardiorespiratory responses to progressive exercise in HTx relative to controls matched for age, sex, body size, and work rate.
Fourteen HTx recipients and matched controls underwent exercise stress echocardiography on a semisupine cycle ergometer. Hemodynamics, left ventricular (LV) dimensions, and volumes were obtained and indexed to body surface area. Oxygen consumption (V˙O2) was measured, and arteriovenous oxygen difference was estimated using the Fick Principle.
At rest, LV mass index (P = 0.03) and volumes (P < 0.001) were significantly smaller in HTx, whereas wall thickness (P < 0.01) and LV mass-to-volume ratio (P = 0.01) were greater. Differences in LV dimensions and stroke volume persisted throughout exercise, but the pattern of response was similar between groups as HR increased. As exercise progressed, heart rate and cardiac index increased to a lesser extent in HTx. Despite this, V˙O2 was similar (P = 0.82) at equivalent work rates as HTx had a greater change in arteriovenous oxygen difference (P < 0.01).
When matched for work rate, HTx had similar metabolic responses to controls despite having smaller LV chambers and an attenuated increase in hemodynamic responses. These findings suggest that HTx may increase peripheral O2 extraction as a compensatory mechanism in response to reduced cardiovascular function.
1School of Kinesiology, University of British Columbia, Vancouver, British Columbia, CANADA;
2Children’s Heart Centre, BC Children’s Hospital, Vancouver, British Columbia, CANADA;
3Department of Pediatrics (Cardiology), University of British Columbia, Vancouver, British Columbia, CANADA; and
4School of Human Kinetics, Trinity Western University, Langley, British Columbia, CANADA
Address for correspondence: Anita T. Cote, Ph.D., Trinity Western University, 7600 Glover Road, Langley, BC, Canada V2Y 1Y1; E-mail: firstname.lastname@example.org.
Submitted for publication June 2018.
Accepted for publication January 2019.